This invention relates to the surface treatment of zinc alloy electroplated steel strips or sheets for outstandingly improving the corrosion resistance thereof and such treated zinc alloy electroplated steel strips.
The zinc alloy electroplated steel strips used herein designate composite zinc-plated steel strips, that is, steel strips having a zinc alloy layer electroplated thereon in which at least one metal is present in addition to zinc, including, for example, steel strips electroplated with Zn-Ni, Zn-Ni-Co, Zn-Ni-Cr, and Zn-Fe alloys.
A variety of electroplated steel strips have been employed in the manufacture of automobiles, electric appliances, and the like. There is a great need for surface treated steel strips having improved corrosion resistance and the demand for such steel strips appears increasing hereafter.
In general, zinc plating has long been used as a typical metal plating for improving the corrosion resistance of steel strips. The zinc plating is to protect steel strips from corrosion by the sacrificial corrosion protection effect of zinc itself. The amount of zinc deposited must be increased in order to enhance corrosion resistance. Increased amounts of zinc deposited, however, not only detract from the workability, weldability, and productivity of zinc plated steel, but also increase the cost. One method for improving the corrosion resistance of such zinc electroplated steel strips is by incorporating an additional metal or metals into the zinc plating to produce zinc alloy plated steel strips. There are well known techniques for electroplating such alloys as Zn-Ni, Zn-Ni-Co, Zn-Ni-Cr, and Zn-Fe.
The alloy plating methods mentioned above are successful to some extent in that since the resulting zinc alloy platings form passivated films effective in retarding or preventing dissolution of zinc, the corrosion resistance of composite zinc plated steel strips is improved by a factor of about 3 to 5 over that of conventional zinc plated steel strips and thus allows the amount of composite zinc platings deposited to be reduced. However, the composite zinc plated steel strips are still liable to formation of white rust and even red rust in relatively short time when they are allowed to stand indoors or outdoors and particularly when they are sprayed with water or salt water.
It was also proposed to carry out a chromate treatment after single or composite zinc plating in order to further improve corrosion resistance. The chromate treatment is effective, but not satisfactory to meet the needs of users in that white rust will appear after about 100 hours under high temperature and high humidity conditions and more under a salt-containing atmosphere.
We previously proposed particular coatings for improving corrosion resistance in Japanese Patent Appln. Kokai Nos. 58-100685 (a composition comprising a polyethylene resin in admixture with a water-soluble chromium compound), 58-153785 (a composition comprising a polyethylene resin in admixture with a melamine resin), and 58-177476 (a composition comprising a polyethylene resin in admixture with colloidal silica). It is zinc electroplated steel strips that are coated with these compositions. The application of these coatings extended the rust prevention time in a salt spray test to about 200 hours. In order to produce surface treated steel strips having further improved corrosion resistance, we have paid attention to zinc alloy plated steel strips because zinc, which offers an active surface liable to white rust formation when used alone, can be converted into a passivated or more corrosion resistant layer by alloying it with nickel, manganese, cobalt or the like. We have found that a synergistic corrosion preventing effect is accomplished when zinc alloy electroplated steel strips are subjected to a proper combination of chromate treatment and coating of a special coating composition.